Part 1: History of the Aluminum Slant Six

At the dawn of the musclecar era, Chrysler engineers were pulling their hair out, replacing hefty iron parts with the svelte aluminum alternatives we've all come to know and love. From alternator housings to automatic transmission cases and fenders to Slant Six blocks, Chrysler led the industry in its quest for increased efficiency through reduced mass. Woah! Did we say aluminum Slant-Six blocks? Few people today realize that nearly 50,000 aluminum-block Slant Sixes were installed in the '61 and '62 model year passenger cars. Today so few survive that most Mopar enthusiasts have never seen one. But that hasn't stopped the rumors from flying, and today they're shrouded in a veil of mystery, misinformation, and confusion.

In this first installment of the Buzzing Half Dozen, we'll clear the smoke and relate the story of how these lightweight aluminum wonders came to be. In our next installment, we'll hammer one together that makes better than one horsepower per cubic inch on 91-octane pump gas. Then we'll stick it in a 3,000-pound A-Body and show you how easy it is to run low fourteens at over 90 mph with any Slant Six, cast iron, or aluminum.

In early 1957, Chrysler launched a program to develop its first compact car to compete with inexpensive foreign invaders like the Volkswagen Beetle and low-line domestic products from Rambler and Studebaker. Word was out that General Motors and Ford were also developing their own compact cars, and the '60 model year was the target release date for what would become the Corvair and Falcon. This gave Chrysler three years to come up with a winner. To have any advantage in the marketplace, Chrysler needed its new small car--code named the A-901--known today as the Valiant, to seat six, carry plenty of luggage, offer exceptional fuel economy, and deliver acceptable performance without becoming too expensive for its penny-pinching target customers. To get these results, a group of approximately 100 designers, draftsmen, engineers, and mechanics were completely segregated from the regular design and engineering staff and put to work in a separate building. Chrysler was banking heavily on its new contender and didn't want its details to leak out prematurely. Under the leadership of Robert M. Sinclair, the Valiant project was started in a brick building located at 403 Midland Avenue in Detroit. The security at the Midland Avenue annex was so tight that many outsiders assumed it housed top-secret military contract work. Many called it the "Mystery of Midland Avenue."

While the A-Body platform was evolving in great secrecy, the engine-development department at Central Engineering faced the reality that Chrysler's 230-cubic-inch flathead six was a heavy, antiquated dinosaur. Designed in 1932, it couldn't deliver the fuel efficiency and power needed in an advanced engineering statement like the Valiant. A deadline of May 1, 1958, was given to the engineers to select a final design configuration for the new Valiant powerplant. Records show they toyed with a 150-cubic-inch inline four-cylinder, a 150-cubic-inch cast iron inline six-cylinder, and even an aluminum V6. All were equipped with overhead valves, but it was a 170-cubic-inch inline six that got the nod. Unlike the upright inline designs under consideration, its novel 30-degree slant allowed for a lower hood line and reduced overall length thanks to its off-center water pump location. These details complemented tight vehicle packaging and were major factors in it being selected for the job of powering the new Valiant.

Aluminum Block Bonanza!

We found three aluminum Slant Sixes in the last few years just by keeping our eyes open. The first was found at the Ontario, California, Pick-A-Part self-serve wrecking yard in April 1998 in the battered '62 Valiant shown here. I bought the entire motor for $135. The second was pulled from a '62 Lancer GT in the San Pedro, California, Pick-A-Part about a year later. This time I disassembled the motor and took only the block, main caps, and main and head bolts for under $100. Both cars must have sat inoperable for years before being sold to the scrap yards, which crushed them after they'd been picked clean of useful parts (typically after 30 days). The first two discoveries are so badly corroded that they are beyond reasonable repair. They still make excellent conversation pieces, though. The third was found at a local machine shop and is a cherry. No corrosion, no cracks, and it still retains a standard bore.

Dare to Compare

Here's an exclusive look (check the pictures at right) at the visual differences that distinguish the aluminum block from its cast iron brother. For easy comparison, these blocks have been stripped of paint, so the shiny die-cast aluminum surface contrasts with the dull cast iron.

Soon after, a request was made for ways to increase displacement to allow enough power for installation in future full-size Dodge and Plymouth C-Bodies and trucks so that the outdated 230-cubic-inch flathead six could at last be retired. The result was a one-inch deck height increase and a two-engine family of 170 and 225 inline six-cylinder engines with a common bore of 3.40 and strokes of 3.125 and 4.125, respectively. On November 26, 1958, less than seven months after the May '58 go-ahead, the first prototype Valiant engine--an all-iron 170--ran under its own power, and the Slant Six was born. Within two weeks, an all-aluminum 170 also sprang to life, and by March 1959, iron and aluminum tall-deck 225s were also among the living.

So what's the deal with the aluminum motors? From the start, Chrysler's intention was to use aluminum for the cylinder block in order to excise dead weight and maximize vehicle efficiency. An aluminum cylinder head was also under consideration, but the cost-to-benefit tally of the light head wasn't nearly as significant as the efficiency bonus offered by a low-mass engine block. After all, the single most bulky item in any car is the engine block, not the head. Although the engineering department did produce some aluminum heads for testing and evaluation (a few of which are still floating around among hardcore Slant Six collectors), economics prevailed, and the aluminum head was never approved for mass production.

An equally significant reason for using aluminum over cast iron was to cut costs. The August '63 issue of Car Life reported that each unmachined aluminum block cost Chrysler $30 to produce--$6 more than cast iron. But this was the only disadvantage. Chrysler was banking on the fact that softer aluminum is easier to machine and doesn't wear out tooling as quickly as cast iron. Chrysler recognized that if tooling life was significantly increased, the capital investment in machinery required to process engine blocks could be reduced. They estimated that three cylinder-block machining lines would be required for cast-iron production, and only two for aluminum. Additionally, by using a recently developed high-pressure die casting process, manufacturing cost could be further reduced. Aluminum parts made with the sand-mold process tend to be heavier for a given size because compensating for core shift and minor discrepancies requires much thicker sections.

Die-casting definitely offers worthwhile advantages. The high injection pressure provides a consistent structure so thinner sections can be used where strength is not critical. Surfaces are smoother and dimensional tolerances closer, so less machining is required to finish the casting. Die-casting is also a lot faster. With no sand cores to set up and shake out, one casting machine can turn out six or seven times as many engine blocks per hour than a permanent mold machine.

The biggest drawback was the up-front cost of the die-casting machines. Though fewer were needed, each weighed a staggering 2,000 tons. The size and weight of the machine was needed to provide the required clamping force to prevent leakage between the dies as the molten metal was injected at 8,000 psi. Still, Chrysler was banking on the increased pieces-per-hour-per-machine to offset the initial investment in manufacturing equipment.

Although the die-casting process was chosen, it was still fairly new to Chrysler. As a precaution against unforeseen interruptions in the supply of aluminum Slant Six blocks, a nearly identical cast-iron engine block was designed in tandem that would be manufactured using the traditional low-pressure, semi-permanent mold-casting technique, and it shared the same 170 low-deck and 225 tall-deck configurations. Quoting Willem Weertman, Managing Engineer of Engine Design (1955-1962): "It was understood that if the development of the aluminum engines did not encounter major problems, the development of the cast-iron versions would be terminated at a future date when the success of the aluminum engines was assured; it was projected that by the start of the '61 model year, both the 170 and 225 engines would consist solely of aluminum blocks.

Because Chrysler banked so heavily on the reliability of aluminum, the changes made to create the "temporary" cast iron version were minimized to save money and time, and to streamline the operation. The ruggedness designed into the aluminum block becomes absolute, bulletproof strength when it is rendered in more stable cast iron. The resulting rigid crankcase endows the approximately 12,500,000 iron Slant Sixes manufactured between 1960 and 1991. Its ability to withstand extreme abuse is the root of the engine's reputation for being virtually indestructible. On the other hand, on the eve of the Valiant's '60 debut, the aluminum Slant Six program was running into trouble.

The Trenton, Michigan, engine plant began producing cast-iron 170 and 225 Slant Sixes in August 1959 using dual-tooled machining lines capable of handling either iron or aluminum blocks. The Canadian Windsor Engine Plant and the Kokomo, Indiana, facility followed within a few months. While the plants began churning out tens of thousands of iron blocks, the production of aluminum blocks was hindered by problems with the cast-iron bore liners and a costly learning curve that led to high scrappage, especially at the Kokomo plant. Although a handful of prototype aluminum block 170s and 225s are reported to have been installed in '60 model year vehicles for analysis in taxi fleets and limited sale to the public (Weertman claims to have leased an aluminum 170-powered Valiant from Chrysler), the production-line kinks weren't ironed out sufficiently to ensure the steady stream required for mass production.

Nonetheless, Chrysler must have been confident the problems had been addressed in August 1960, when it issued PTR (Production Test Request) No. 3331. It authorized the building of 15,000 aluminum-block 225 engines for installation in '61 model year vehicles, and records indicate that an actual total of 10,767 aluminum block Dodge and Plymouth cars were built. (Oddly, the 170 was not included in this production run, and all '61-'69 170s used iron blocks.) The '61 PTR was encouraging enough for Chrysler to give the aluminum-block 225 a regular production release for the '62 model year, and approximately 36,000 more were installed as optional equipment in Valiants and Lancers. We have not been able to confirm whether any regular-production full-size C-Bodies or trucks were ever built with aluminum blocks. Despite the assembly and sale of nearly 50,000 aluminum-block Slant Six vehicles in 1961 and 1962, the die-casting process failed to achieve the problem-free non stop processing required for consistent high-volume manufacture. And while the reduced weight of the aluminum block was highly praised by magazine road testers from Hot Rod, Motor Trend, and others, the benefit was lost on the buying public. At the end of the '62 model year, production of aluminum blocks was discontinued.

So, what happened to the roughly 50,000 aluminum-block 225s released into circulation? The vast majority suffered from corrosion. Because the aluminum block was an extra-cost option (see sidebar), it is safe to assume that most original owners were diligent about following Chrysler's directions to flush the coolant twice a year and use a rust inhibitor. But as the cars were traded in and entered into the used-car market, their second, third, and fourth owners were often oblivious to the presence of the special aluminum block and its tendency to corrode from the inside out when neglected. As a result, galvanic activity quickly took its toll around the tops of the free-standing cylinder barrels until the head gasket blew. Unfortunately, once this level of corrosion was reached, chronic head-gasket failure resulted, and nothing short of expensive and risky weld repair work could reverse the damage. Add to this the unpopular style of the cars that housed the aluminum blocks, and it's likely that most met the crusher by 1975.